Recently, an organic reaction catalyzed by metal/metal oxide nanoparticles (NPs) has attracted much attention. The remarkable advantages of this family of heterogeneous catalysts, such as high catalytic activity with improved selectivity and good recyclability, extend its use to a wide-range of applications in various organic reactions.
Metal-oxide nanoparticles such as TiO2 and ZnO, serve many functions in polymeric industry. Traditionally, theses nanoparticles have been used as pigments to increase the appearance and durability of polymeric products. As nanosized particles, of these materials exhibit broad band UV absorption, the use of the same is extended to cosmetic applications.
In view of the significance, metal-oxide/metal nanoparticles are also useful for a large variety of more sophisticated applications including use in catalysis, as sensors, optoelectronic materials and in environmental remediation.
The conventional prior art preparation techniques for Metal oxides (MOx) NPs typically use organometallic precursors to form NPs with diameters greater than 2 nm. With the prior art methodology, only larger structures such as nanorods, nanotubes, nanoneedles, and nanowires have been reported in literature.
Further to dissolve the metal precursor surfactant is employed in the art, couple of the metal-surfactant complex is disclosed herein below.
KR100967708 (BAEK et al.) discloses a process for producing metal oxide nano-particles; wherein the process comprises, adding of a surfactant to a dispersing organic solvent and mixing to prepare a surfactant solution; mixing the surfactant solution and a metal salt water solution to prepare a first oil-in-water type emulsion, wherein the metal of the metal salt is selected from iron, cobalt, nickel, and chromium; mixing the surfactant solution and a basic solution to prepare a second oil-in-water type emulsion, mixing the first and the second oil-in-water emulsions to prepare a metal oxide nano-particle colloid solution by reacting the metal salt and the basic solution.
U.S. Pat. No. 7,407,527 (Hyeon Taeghwan et al.) discloses a method for producing metal or metal alloy nanoparticles, comprising the steps of; forming a metal-surfactant complex by reacting a metal precursor and a surfactant in a solvent, further synthesizing monodisperse metal nanoparticles by thermally decomposing. Similarly U.S. Pat. No. 6,572,673 discloses a process for preparing metal nanoparticles, comprising reacting suitable metal salts and anionic surfactant containing an anionic group of carboxylic group sulfate group or sulfonate group as reducing agent in water under reflux at a temperature of 50-140° C. followed by reduction to afford nanoparticles.
Additionally, the preparation of monodisprese metal nanoparticles by polyol method is reported in US 20070056402 (Cho Sung-Nam Et Al.), and article like Nanostructured Materials 11 (8), November 1999, Pg. 1277-84; and U.P.B. Sci. Bull (D Berger), series b, vol. 72, iss. 1, 2010. However in polyol method the morphology and dimension of metal nanoparticles is strongly depend on reaction condition, which leads to irregularity in shape.
Mathias Brust et al., in J. Chem. Soc., Chem. Commun., 1994, 801-802 discloses synthesis of thiol-derivatised gold nanoparticles by two-phase (water-toluene); wherein AuCl4− was transferred from aqueous solution to toluene using tetraoctylammonium bromide as the phase-transfer reagent and reduced with aqueous sodium borohydride in the presence of dodecanethiol to obtain solutions of 1-3 nm gold particles bearing a surface coating of thiol.
However, use of phase transfer, cannot be standardized across metals where monodispersibility of metal may get affected.
WO2012009070 discloses a method of making ultra small metal oxide nanoparticles by placing water soluble, inorganic ammonium oxometalate precursor in a reactor; adding an excess of amine surfactant to said reactor, optionally adding diols or amine oxides to said reactor; heating the reactor until the ammonium oxometalate precursor structure collapses and the nucleation stage generates ultrasmall metal oxide nanoparticles of average size<5 nm.
Researchers have attempted to prepare metal nanoparticles by different methods. One such method includes reduction of palladium salt in a solution in situ in presence of a reducing agent such as easily oxidized alcohols and a stabilizing agent such as PVP to obtain PVP-Pd nanoparticles. Use of such stabilizing agents prevents the agglomeration of the nanoparticles. (CHEM 7530/750, WINTER by Olivier Nguon).
Deshmukh and co-workers carried out the Heck reaction using Pd(OAc)2 and PdCl2 catalysts in 1,3-dibutylimidazolium bromide {[BBIm][Br]} IL under ultrasonic irradiation conditions and reported the formation of 20 nm Pd nanoparticles composed of 1 nm nanoclusters formed via reduction of Pd2+ ions during catalytic reaction. Since ionic liquids are expensive, use of the same escalates the cost of the process.
References may be made to patent U.S. Pat. No. 6,572,673, wherein nanoparticle synthesis and the size control is achieved by adjusting the rate of reduction. This has done by using appropriate reducing agents, nature of surfactants and/or the concentration of surfactants, temperature, rate of addition, cleanliness of glass ware, amount of reagents used, volume of the vessel etc. Any small change in any of the conditions listed above leads to the formation of larger particles or broader size distributions. In our procedure we will not face this problem as we start with a poly disperse system and CONVERT that to a mono disperse system.
A cursory review of the prior art reveals that there is no report for the synthesis of transition metal nanoparticles of small sizes (2-3 nm) in organic medium. Thus there is a need to provide a robust process for the preparation of transition metal nanoparticles that can be easier for industrial scale up.
In accordance with the need the present inventor has developed one pot process for the preparation of ultra-small uniform sized metal nanoparticles exhibit better catalytic activity and shape tunabillity.